Soybean protein has been utilized as an excellent source of food protein for a long time. In addition, since soybean protein has various functional properties such as emulsifying property and gel-forming property, it has been widely used as a food material or a food-improving material for edible meat products, fishpaste products, side dishes, bread, confectionery and a raw material for beverage. Further, it has been recently found that soybean protein reduces a blood cholesterol level and then its nutritional and physiological functions have been drawn attention.
Conventional means of positively taking soybean protein are “side dishes” such as tofu (soybean curd), fried tofu, natto (fermented soybeans) and the like. In addition, there is a type of soybean protein foods such as protein powder and a protein bar. However, these foods are all in the neutral range. There are no dessert foods with an acid pH, for example, a dessert prepared using a combination of soybean protein and fruit juice. Thus, means of eating soybean protein are limited. Among dessert foods, jelly can provide enjoyable mouth feeling and throat feeling and thus can provide an eating scene different from beverage.
Foods prepared by utilizing the gelation property of soybean protein include yuba (soybean curd skin) and frozen tofu in addition to the above-mentioned tofu, and they have been traditionally eaten for a long time. A gel of soybean protein has characteristics different from a gel of other protein materials derived from milk or eggs and a gel of polysaccharides such as carrageenan or agar in the mouth feeling, gel properties, nutrition and the like. However, traditional foods such as tofu are prepared by utilizing the gelation property of soybean protein in the pH range form neutral to lower acid (i.e. a pH above the isoelectric point of soybean protein) and at present there are no foods prepared by utilizing the gelation property of soybean protein in the acid range (below the isoelectric point), for example, gel dessert foods prepared using a combination of soybean protein and fruit juice.
Although soybean protein typically has an excellent gelation property in the neutral range, it does not exhibit the gelation property in the acid range. This is due to the following reason. Soybean protein dissolves in the neutral range and then exhibits the gelation property. On the other hand, at below pH 4.6, i.e. the pH range (pH 3.0 to 4.5) of so-called acidic foods (see the non-patent document 1), soybean protein hardly dissolves and then can not exhibit the gelation property. This is due to the pH of acidic foods equal to or in the vicinity of the isoelectric point (about pH 5) of soybean protein. Thus, many processed foods containing a high concentration of soybean protein which can provide soybean protein efficiently have been limited to foods with a neutral or slight acid pH as described above. Therefore, soybean protein foods prepared by utilizing the gelation property of soybean protein in the acid range are extremely expected to increase means of taking soybean protein and diversify soybean protein foods.
Many of conventional techniques relating to utilization of soybean protein for acidic foods are mainly directed to prevention of the aggregation and/or precipitation of soybean protein in the acid range in producing acidic beverages. For example, an addition of a stabilizer such as pectin (see the patent document 1), an addition of an emulsifier such as a sucrose fatty acid ester having an HLB of 13 or more (see the patent document 2), a method of suppressing the aggregation of soybean protein due to passing through the isoelectric point (see the patent document 3 and 4) and the like are known. However, these methods do not make the protein itself soluble. Thus, the soybean protein treated with these methods does not exhibit the functional properties of the protein itself such as the emulsifying property and the gel-forming property, let alone provide a clear gel, and thereby a type of foods for which these methods are used is limited. In addition, in order to minimize the aggregation of the protein due to passing through the isoelectric point, the concentration of the protein needs to be lower.
As described above, although soybean protein foods prepared by utilizing the gelation property of soybean protein in the acid range have been potentially in great demand, a method of allowing soybean protein to gelate in the acid range has not be found because of the poor solubility of soybean protein in the acid range and thus, to date such foods have been not produced.
The present invention is to provide gel foods of soybean protein, especially jelly-like foods, prepared by utilizing the gel-forming property of soybean protein in the acid range, which provide a variety of eating habits in taking nutritionally excellent soybean protein, and to provide a process for producing the same.